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Neutron Sources & Energies

Neutrons are classified according to their energy. The following is a listing of the terms used to describe the various classifications, followed by the basis for each term, and then the range of neutron energies that defines the class:

Thermal

  • Neutrons in thermal equilibrium with their surroundings
  • Most probable energy at 20 degrees (C) – 0.025 eV; Maxwellian distribution of 20 degrees (C) extends to about 0.2 eV.

Epithermal

  • Neutrons of energy greater than thermal
  • Greater than 0.2 eV

Cadmium

  • Neutrons which are strongly absorbed by cadmium
  • Less than 0.4 eV

Epicadmium

  • Neutrons which are not strongly absorbed by cadmium
  • Greater than 0.6 eV

Slow

  • Neutrons of energy slightly greater than thermal
  • Less than 1 to 10 eV (sometimes up to 1 keV)

Resonance

  • In pile neutron physics, usually refers to neutrons which are strongly captured in the resonance of U-238, and of a few commonly-used detectors (e.g., Indium, Gold, etc.)
  • 1 eV to 300 eV

Intermediate

  • Neutrons that are between slow and fast
  • Few hundred eV to 0.5 MeV

Fast

  • Greater than 0.5 MeV

Utrafast

  • Relativistic
  • Greater than 20 MeV

Pile

  • Neutrons of all energies present in nuclear reactors
  • 0.001 eV to 15 MeV

Fission

  • Neutrons formed during fission
  • 100 keV to 15 MeV (Most probable – 0.8 Mev; Average – 2.0 MeV)